A porous film having fine through holes is used as a filter medium for microfiltration so as to filter impure particles in a liquid or a gas in various fields such as medical, semiconductor manufacturing and food industry fields. In the case where the gas or liquid treated is an organic solvent, some filters may exhibit corrosivity to the organic solvent or may be used in a high-temperature environment. In such a case, chemical resistance, chemical stability, heat resistance and the like are often required of the filter. As a material meeting this requirement, a porous film formed of a fluorine-based resin has been developed. However, the fluorine-based resin is expensive and furthermore, highly hydrophobic and therefore, there is a problem that in the case of filtering an aqueous treating solution, a hydrophilization treatment must be previously applied to the fluorine-based resin-made filter medium or the filter medium must be immersed in an alcohol before use.
On the other hand, a porous film is also being used as a separator that is a constituent member for preventing contact between an anode and a cathode in a lithium ion secondary battery, an electrical double layer capacitor, an electrolytic capacity and the like. Recently, in view of safety and product life, the requirement for heat resistance of the separator is increasing. As for the separator of a lithium ion secondary battery used at present, a porous film made of polyethylene or polypropylene is being predominantly employed. However, the polyethylene and polypropylene are poor in the heat resistance, and there may be a possible risk that a separator using such a resin is melted/softened at a high temperature and shrinks to put the anode and the cathode into contact and cause a short circuit. In an electrical double layer capacitor and an electrolytic capacitor, paper formed of a cellulose material is being predominantly used. However, in this case, an electrolytic solution developed for a high temperature-resistant application, where an ion liquid such as 1-ethyl-3-methylimidazolium tetrafluoroboric acid is dissolved as an electrolyte in a solvent such as γ-butyrolactone, decomposes or dissolves the cellulose at a high temperature, giving rise to a problem that the life of the product is short.
An aliphatic polyketone (hereinafter, sometimes referred to as polyketone) obtained by polymerizing carbon monoxide and an olefin by using palladium or nickel as a catalyst, where the carbon monoxide and the olefin are completely alternating copolymerized, is known. Due to its high crystallinity, the polyketone when formed into a fiber or a film exhibits characteristics such as high mechanical property, high melting point, organic solvent resistance and chemical resistance. Above all, when the olefin is ethylene, the melting point of the polyketone is 240° C. or more. Such a polyketone is excellent in the heat resistance as compared with, for example, polyethylene. Accordingly, a polyketone porous film obtained by processing the polyketone into a porous film also has heat resistance and chemical resistance. Furthermore, because the polyketone has affinity for water and various organic solvents and also, the carbon monoxide and ethylene as raw materials are relatively inexpensive, making it possible to lower the cost of polymer of the polyketone, the polyketone porous film is expected to find its utilization in industry as a filter medium. In addition, when the polyketone porous film is flat film-shaped, the porous film can also be used as a separator capable of solving the above-described problems of a lithium ion secondary battery and various capacitors and therefore, is more useful.
Usefulness of the polyketone porous film as a filter medium is described, for example, in Patent Document 1 and Patent Document 2.